Copy std::map into std::vector of pairs

问题

I'm trying to copy a map into a vector of pair, so I can then sort the vector by the second data member of the pairs. I have resolved this doing like this:

void mappedWordsListSorter(){
  for (auto itr = mappedWordsList.begin(); itr != mappedWordsList.end(); ++itr){
    vectorWordsList.push_back(*itr);
  }
  sort(vectorWordsList.begin(), vectorWordsList.end(), [=](pair<string, int>& a, pair<string, int>& b){return a.second > b.second;});
}

I need to find a way to do this without using a raw loop, using the standard library instead. I have come across a lot of examples doing this by only transferring either the keys or the values of the map. I need to copy into a vector of pairs<string, int>. What is the best way to do it?

回答1:

Just use std::vector's assign member function.

//no need to call reserve, bidirectional iterators or better will compute the size and reserve internally.
vectorWordsList.assign(mappedWordsList.begin(), mappedWordsList.end());

If you have existing values in the vector that you don't want overwritten then use insert instead like

vectorWordsList.reserve(vectorWordsList.size() + mappedWordsList.size()); // make sure we only have a single memory allocation
vectorWordsList.insert(vectorWordsList.end(), mappedWordsList.begin(), mappedWordsList.end());

回答2:

You can use std::copy and std::back_inserter:

std::copy(mappedWordsList.begin(), 
          mappedWordsList.end(), 
          std::back_inserter(vectorWordsList));

Honestly, I think that a range-for loop is clearer:

for(const auto& kv : mappedWordsList) 
     vectorWordsList.emplace_back(kv);

Regardless, you can use std::vector::reserve to preallocate memory on your target vector, avoiding unnecessary reallocations.

回答3:

It's worth noting that if you are creating a vector for this purpose, you may use the vector's constructor directly:

std::vector<std::pair<FirstType,SecondType>> vectorWordsList( mappedWordsList.begin(), mappedWordsList.end() );

In C++17, you may also omit the vector's template arguments to have the compiler deduce them:

std::vector vectorWordsList( mappedWordsList.begin(), mappedWordsList.end() );

来源：https://stackoverflow.com/questions/53854468/copy-stdmap-into-stdvector-of-pairs